fix(skills): accept interoperable allowed-tools forms

[jmagar]

Summary

• accept non-empty YAML-array allowed-tools frontmatter in addition to the normative Agent Skills string form
• parse comma-delimited and whitespace-delimited string forms into non-authorizing tool_hints without splitting spaces inside parenthesized tool patterns
• preserve raw frontmatter verbatim for SEP cross-verification and document the compatibility behavior

Why

Live Labby was excluding valid Skills emitted by real Agent Skills tooling as invalid_frontmatter. Dendrite also uses comma-delimited strings that skills-ref accepts, while Labby previously tokenized them into bogus names such as Read,. SEP-2640 already requires allowed-tools to remain a hint rather than an implicit permission grant.

Verification

• cargo fmt --manifest-path crates/labby-runtime/Cargo.toml -- --check
• cargo clippy -p labby-runtime --all-features --lib --locked -- -D warnings
• cargo test -p labby-runtime --all-features --locked skills:: → 90 passed
• cargo test -p labby-runtime --all-features --locked --test skills_contract_conformance → 8 passed
• cargo test -p labby-runtime --all-features --locked --test sep_2640_uri_conformance → 11 passed
• cargo test -p labby-gateway --all-features --locked skills → 84 passed

cargo clippy -p labby-runtime --all-features --all-targets --locked -- -D warnings is independently red on pre-existing unused_async_trait_impl warnings in agent_runtime.rs test fixtures, outside this diff.